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Introverts and Type A workaholics may be more vulnerable to recurrent depression than people with active social support systems.

If you tend to prefer solitude to a social setting, you may be more susceptible to recurrent bouts of depression than are those who frequently surround themselves with friends and acquaintances.

Similarly, if you're a Type A personality who channels aggression by being productive in the workplace, you may be more likely to suffer repeat attacks of depression than people who are very personable.

In a study by psychologists at the University of Washington in Seattle, researchers tracked 78 patients who had made a full recovery from depression at least two months before. The patients had all received 20 sessions of cognitive behavioral therapy, a type of treatment that helps people learn new ways to prevent destructive thoughts or behavior.

Research has shown that between 50 and 80 percent of depressed patients successfully treated with cognitive behavioral therapy suffer a relapse, typically within two years.

At the end of the study, 34 people, or 44 percent, had relapsed. The researchers said the loner personality was more likely to be in this group because they tended to have little or no social support system.

"Low dependency increases risk for relapse, while moderate dependency encourages recovered patients to seek out social relationships that may function over time to reduce relapse risk," said researcher Jackie Gollan.

People with aggressive personalities were also believed to be more at risk for depression because they tend toward isolation, a characteristic that may also be a risk factor if you have depression, said Gollan.

Additionally, Gollan's team found that those at risk of relapse were less likely to report high levels of satisfaction in participating in activities they enjoyed, compared to those at lower risk.

"We don't know why, but it's clear that people are less at risk for relapse when they do things they enjoy, rather than working on overcoming their negative thinking patterns," she added.